Enantiomerically pure compounds are of great interest in the pharmaceutical industry and other fields. The rapid and efficient separation and collection of chiral compounds is difficult since they have the same physical properties. All of their physical properties correspond, except the direction in which they rotate plane-polarized light, i.e. they differ in a specific optical activity. Furthermore, all of their chemical properties correspond, except the reactivity toward other chiral compounds. Note that the term chiral compounds are also often used as a general term that refers to the molecules with a chiral center. Development of both preparative and analytical scale separations has provided the tools to determine the enantiomer composition for racemic mixtures, further establishing evidence of the enantiomer rates of activity. Although it has also extended into the agrochemical and food industries, this technology has been primarily driven by the pharmaceutical industry.
The processes traditionally employed for enantiomer preparation, however, suffer from several drawbacks. For example, one process is liquid or gas chromatography. In this process, the analysis mixture is mixed with an externally prepared carrier medium and separated in a separating column as a function of the different affinity of the enantiomers for the stationary phase of the chromatographic column; and thus, the individual components pass in succession through the chromatography column as a function of their different retention times. This process, however, can be very time consuming when multiple samples (such as might be desired in high-throughput screening) are to be analyzed as elution times of 20-30 minutes for one sample, are relatively common. A further disadvantage of the chromatographic process is that the enantiomeric molecules can often have very similar retention times, leading to poor separation per pass.
One of two approaches is typically utilized for chiral separation: 1) indirect and 2) direct separation methods. Indirect separation methods incorporate a reaction between each enantiomer and a chiral molecule to covalently form a new complex, which is then separated from the other enantiomeric complex. This approach is frequently utilized, especially in large-scale operations. Direct methods are based on the formation of non-covalent diastereomeric pairs of molecules using a chiral selector (CS) and rely on differences in the energetics of the complex formation for enantiomer resolution. The chiral selector can either be incorporated into the stationary phase or as an additive in the mobile phase. Chromatography and capillary electrophoresis (CE) have been primarily exploited for chiral separations, both prep-scale and microscale. Typically in chromatography, the stationary phase is chiral (CSP) but chiral additives may also be added to the mobile phase (in liquid chromatography). The first analytical separation of two enantiomers occurred with gas chromatography, but due to the required analyte volatility for gas chromatography, its applications are limited. It is for this reason that liquid chromatography is more commonly employed. In CE, a chiral selector (CS) is added to the electrolyte solution.
Both CE and HPLC have received considerable attention, however, a major difficulty with both techniques is that prediction of the separation conditions remains difficult. For example, in HPLC, there are over 200 CSP's commercially available, yet no clear method to determine which CSP will provide a good separation. This can lead to both time-consuming and costly method development. The fact that HPLC and sometimes CE require longer analysis times (minutes to hours) combined with the lengthy method development creates a real need for analytical tools which either are predictable in the separation capabilities or have faster analysis times, specifically in the early stages of drug development.